1. Field of the Invention
The present invention relates to a collimated sputtering method and a system used therefor and more particularly, to a collimated sputtering method applicable to formation or deposition of various thin films for semiconductor device fabrication process, and a collimated sputtering system used for the method.
2. Description of the Prior Art
Recently, the integration level of semiconductor integrated circuit devices has been increasing, as has the use of three-dimensional device structures. As a result, contact holes, which are used for electrically connecting an interconnection film or films with underlying conductor regions through an insulator film or films, have had a higher aspect ratio. The aspect ratio is defined as a ratio of a depth of the hole to a diameter thereof.
A titanium (Ti) film, when directly contact with a silicon substrate or layer, produces a titanium silicide (TiSi.sub.2) film during a heat-treatment process to thereby reduce and stabilize its contact resistance. Accordingly, the Ti film has been popularly employed for the interconnection film.
However, as shown in FIG. 1, a Ti film produced by a conventional non-collimated sputtering method has a very low bottom coverage. This bottom coverage decreases to 10% or less with the increasing aspect ratio of the contact holes.
For example, as shown in FIG. 2, an insulator film 32 such as a silicon dioxide (SiO.sub.2) film having a penetrating contact hole 32a is formed on a single-crystal silicon substrate 31. The hole 32a has a diameter of d and a depth of h and therefore, an aspect ratio of the hole 32a is expressed as (h/d). Since h is much greater than d, the aspect ratio is very high.
A titanium (Ti) film 33 is formed on the insulator film 32 to cover the contact hole 32a. A titanium nitride (TiN)film 34 is formed on the Ti film 33, and a tungsten (W) film 35 is formed on the TiN film 34. The Ti, TiN and W films 33, 34 and 35 constitute an interconnection conductor to be electrically connected to the substrate 31 through the contact hole 32a.
If the Ti film 33 has a large thickness (for example, 10 nm) to ensure a sufficient thickness of the Ti film 33 at the bottom of the contact hole 32a, the top opening of the hole 32a becomes narrow due to the deposited Ti film 33 on the sidewall of the hole 32a. This hinders the subsequently deposited TiN and W films 34 and 35 from burying the hole 32a.
Also, even if the thickness of the Ti film 33 is increased until the top opening of the contact hole 32a is closed by the Ti film 33, the thickness of the Ti film 33 at the bottom of the hole 32a does not become sufficiently large. This means that no good contact characteristic is obtained.
To solve the above problem relating to the bottom coverage in the contact hole 32a, a collimated sputtering method was developed, which was disclosed in the Japanese Examined Patent Publication No. 6-60391 published in August 1994 and the Japanese Non-Examined Patent Publication No. 1-116070 published in May 1989. In this method, a plate-like collimator having a large number of penetrating apertures is used. The sputtered species or atoms of a target travel through the apertures of the collimator to the substrate.
The collimator traps the sputtered species travelling in the direction tilted at large angles with respect to the normal direction to the sputtering surface of the target (i.e., the surface normal direction), thereby controlling or limiting the directionality of the sputtered species. Thus, the rate of the travelling sputtered species that can reach the bottom of the contact hole increases to thereby improve the bottom coverage, which is shown in FIG. 1.
FIG. 1 shows the aspect ratio dependency of the bottom coverage curve of the conventional collimated sputtering method in which the apertures of the collimator has an aspect ratio of 1.5. Similar to the case of the contact holes, the aspect ratio of the apertures of the collimator is defined as a ratio of a depth of the aperture to a diameter thereof. If this value of the aspect ratio is further raised, a higher bottom coverage is obtained, because the collimator allows the sputtered species travelling in the directions tilted at smaller angles to the surface normal direction to pass therethrough.
With the above conventional collimated sputtering method, the angular distribution of the sputtered species was not fully considered to make it suitable to the collimated sputtering. In other words, the angular distribution was not optimized for this purpose.
Specifically, with the above conventional collimated sputtering method disclosed in the Japanese Examined Patent Publication No. 6-60391, the discharging voltage, which is a voltage applied across the substrate and the target for generating a glow discharge, is 500 to 600 V. Therefore, the rate of the sputtered species travelling along the surface normal direction is comparatively low. This means that the rate of the trapped species by the collimator is high; in other words, the rate of the species reaching the substrate is low.
For example, when a collimator with apertures having an aspect ratio of 1 is used, the rate of the species reaching the substrate through the collimator decreases to approximately a fifth (1/5) as much as that when no collimator is used. In this case, it can be said that a film is deposited on the collimator rather than on the substrate.
FIG. 3 shows the angular distribution of the sputtered species as a function of the tilted angle with respect to the surface normal direction, in which the applied voltage for discharge is used as a parameter. As seen from FIG. 3, when the discharging voltage is as low as 500 V, the relative number of the species in a direction tilted at 45.degree. with respect to the surface normal direction of the target and that in a direction tilted at an angle of zero (0) (i.e., parallel to the surface normal direction) are approximately 5:3.
The sputtered species that are trapped by the collimator narrow the paths of their apertures and therefore, the sputtering or deposition rate decreases with the increasing repetition number of use, which requires frequent exchange of the collimator. This means that the lifetime of the collimator is short. Such a problem becomes remarkable when the collimator apertures have a high aspect ratio in order to obtain a higher bottom coverage.
Further, with the above conventional collimated sputtering method, the same target as that used for the conventional non-collimated sputtering method has been used. The relationship between the sputtering surface of the target and the direction of its crystal axes was not considered.
For example, for a polycrystalline titanium target that has been used in the conventional collimated sputtering method, the approximately 80% grains of the target have a (0001) plane on the sputtering surface. Therefore, this target has an angular distribution of the sputtered species as shown in FIG. 4 in which the relative number of the species is relatively low in the surface normal direction and relatively high in the oblique direction thereto. The peak of the relative number is tilted at approximately 35 degrees with respect to the surface normal direction. The relative number at the peak angle and that in the normal direction is approximately 2:1.
As a result, the relative number of the sputtered species trapped by the collimator is large and therefore, the relative number of the species reaching the substrate is small. For a collimator with the aperture aspect ratio of 1, the relative number of the species reaching the substrate through the collimator decreases to approximately (1/5) as much as that when no collimator is used. In this case, it can be said that a film is deposited on the collimator rather than on the substrate. This leads to a low growth or deposition rate per applied unit power.
Additionally, with the above conventional collimated sputtering method, a satisfactorily high bottom coverage cannot be obtained corresponding to a high aperture aspect ratio of the collimator. This is caused by the angular distribution of the travelling, sputtered species.
Specifically, the collimator does not allow only the species travelling in a direction exactly parallel to the surface normal direction to pass. The collimator allows the species travelling in directions inclined to the surface normal direction within a specified angle (for example, 33.7 degrees or larger for the collimator aspect ratio of 1.5) to pass also. Accordingly, if the relative number of the species near the specified angle is high and that near the surface normal direction is low within the angle range enabling the species to reach the substrate, good bottom coverage cannot be obtained.
The Japanese Non-Examined Patent Publication No. 5-29257, which was published in February 1993, disclosed a concept of controlling the distribution of the travelling sputtered species with respect to the sputtering surface of a target, thereby improving the shape of a deposited film in a recess or hole.
The Japanese Non-Examined Patent Publication No. 59-104476, which was published in June 1984, disclosed a concept of controlling a discharge voltage in response to drop of the voltage to thereby stabilize the plasma and deposition rate during a sputtering process.